The present invention relates to a method for monitoring BCCH carriers in GSM, and in particular to a method to control monitoring with discontinuous BCCH carriers in GSM.
In recent years, mobile telecommunication systems have become increasingly popular based on usefulness and commonly availability. Moreover, a continued increase in the need for such systems and additional services associated therewith is expected for the near future.
One of the most widely spread and presently available mobile telecommunication systems is the so-called GSM system (xe2x80x9cGroupe Speciale Mobilexe2x80x9d) which meanwhile has become a common standard in European countries.
In general, in a mobile telecommunication system such as the GSM system, a mobile station MS, whether in a standby mode or in use, has to continuously communicate with a base transceiver station BTS or base station BS, respectively, in order to provide at any time the possibility to establish a radio link, i.e. to initiate or receive a call.
In connection with such a communication between the mobile station MS and the base station BS, information relating to the so-called radio link control process, like data necessary for control processes like power control (PC) of the used transmitters and handover (HO) of the mobile station MS from the currently associated base station BS to a neighbouring base station, are acquired, transmitted and evaluated. These above mentioned control processes performed in connection with the radio link control process are managed and effected by a so-called base station controller BSC superordinated to the base stations.
To this end, according to the GSM standard, the mobile station MS is required to monitor the received power levels of up to 32 neighbouring base stations BS and to transmit the received power levels of the six strongest neighbours and the corresponding signal quality to the base station BS it is presently allocated to.
Those measurements on the radio interface are effected for each radio link of a mobile station MS on a signalling channel and/or traffic channel associated to the respective mobile station MS, i. e. for each radio link between a mobile station and a base station BS it is presently allocated to and neighbouring base stations.
As to the transmission system according to GSM, it is distinguished between logic channels of different kinds, namely, between so-called traffic channels (TCH) and signalling channels also referred to as control channels (CCH).
Traffic channels (TCHS) are used for transmitting speech or data. The traffic channels have data rates of 13 kbit/s (full rate channel) or 6.5 kbit/s (half rate channel) for speech transmission, and a data rate between 2.4 kbit/s and 9.6 kbit/s for data transmission (at full rate) or up to 4.8 kbit/s (at half rate).
Control channels (CCHs) are used for transmitting control signals required for establishing and/or maintaining a radio link between the mobile station MS and the base station(s) BS. Among the control channels different types of control channels are distinguished, namely, e. g.
Broadcast CHannels (BCH),
Common Control CHannels (CCCH), and
Dedicated Control channels (DCCH).
Broadcast channels BCH are directed from a base station BS to a mobile station MS in a so-called downlink. Within a broadcast channel BCH it can be distinguished between
a Frequency Correction CHannel (FCCH) for frequency synchronization of the MS with the BS,
a Synchronization CHannel (SCH) for subsequently effecting a bit-synchronization of the MS with respect to the BS (i.e frame synchronization), and
a Broadcast Control Channel (BCCH) used for transmitting the basic information required by the mobile station MS for communication with the base station(s) BS. After all synchronizations have been effected, the MS can evaluate all received BCCH information necessary for the communication.
The Common Control CHannel (CCCH) is used for radio link establishment and includes a paging channel (PCH) and an access grant channel (AGCH) in downlink direction, and an random access channel (RACH) in uplink direction.
The dedicated control channels include a stand alone dedicated control channel (SDCCH) and an Associated Control CHannel (ACCH), the latter of which comprises the Fast Associated Control CHannel (FACCH) and the Slow Associated Control CHannel (SACCH), respectively accompanying a traffic channel TCH and used in connection with an assignment of a TCH or at handover (FACCH) or for measurement result transmission (SAACH).
The channels briefly described above each are logical channels which are mapped to physical channels of the GSM radio frequency band, respectively, with the GSM radio frequency band being divided in a downlink frequency band for BS to MS communications and an uplink frequency band for MS to BS communication.
Now, with respect to the data transmission method used on the respective (physical) channels, due to the fact that a mobile station MS uses only part time for transmitting/receiving of information and uses remaining time for effecting measurements of the kind briefly described above, a channel (carrier) assigned thereto is not continuously used by the mobile station MS for communication.
Therefore, a (physical) channel or carrier, respectively, has been made available to other stations for data transmission during the time (a first) mobile station MS effects measurements, by adopting the TDMA method (Time Divisional Multiple Access) for data transmission on the respective physical channel.
Thus, a mobile station uses a physical channel or carrier, respectively, intermittently, and the base station may communicate continuously with several mobile stations using a single carrier. Alternatively, a BS may communicate with several mobile stations MS using multiple carriers in a case of frequency hopping, for example.
To state this in greater detail, in mobile communication systems adopting the TDMA method a time-divisional communication takes place on the respective channel or carrier frequency, respectively, in consecutive so-called TDMA frames. A single TDMA frame consists of several so-called time slots TS of predetermined duration during which different contents of information referred to as a burst is transmitted. In particular, according to GSM standard, a time slot TS has a duration of 576,9 xcexcs and eight consecutive time slots constitute a TDMA frame having a duration of 4.615 ms. Furthermore, the TDMA-frames consisting of eight time slots TS each are grouped to form multi-frames consisting of 51 TDMA-frames in case of a control channel CCH and 26 TDMA-frames in case of a traffic channel TCH.
The general concept of this conventional (Prior Art) hierarchy briefly described above is schematically depicted in FIG. 1 of the accompanying drawings.
Within the respective time slots TS of the frames information is transmitted as the above mentioned so-called bursts. Five different types of bursts are defined, namely
normal burst NB
frequency correction burst FB
synchronization burst SB
dummy burst DB, and
access burst AB.
The FB, SB and AB burst, respectively, are bursts transmitted when a radio link is being established. After a link has been established, the further communication and exchange of information is effected using normal bursts. The dummy burst is transmitted when a transmission is not effected in all time slots of the BCCH carrier (the so-called beacon carrier).
Thus, one physical channel or carrier, respectively, supports a minimum number of at least 8 logical channels (1 logical channel corresponding to one time slot) (or 16 logical channels in case of the above mentioned half-rate transmission). Stated in other words, a single carrier supports a couple of connections. e. g. 8 connections, simultaneously, one for BCCH and the remaining for TCHs.
On the respective broadcast control channel BCCH general information concerning the base station is transmitted and the mobile station must measure the received power of the BCCH carrier of the neighbouring base stations and decode the BCCH signal of the neighbouring cells.
To this end, according to the GSM specification, the carrier on which the BCCH channel resides must be transmitted continuously with constant power and at a standard frequency. In particular, the BCCH carrier on which the BCCH channel is transmitted has to be transmitted in all time slots, since the measurement of the received signal level of the neighbour cells of the MS rely thereon.
As already stated hereinabove, in current BTS the minimum number of channels is 8, namely 1 for BCCH and the rest for TCHs.
However, in various future scenarios the GSM BTS is required to support less than 8 connections simultaneously. For example in scenarios like Home Base Station of Office BTS, the presently available capacity of 8 simultaneous connections exceeds the need, thus being a non-optimum solution. For those scenarios an optimum solution has only one or two traffic channels TCHs. Consequently, this requires the use of only two time slots TS out of the eight time slots provided in the carrier and/or channel to be used for transmission.
In particular, those so-called dual slot BTSs are, by definition, not able to transmit on all time slots, but only in two or three out of eight, thus violating the GSM specifications requiring the BTS to transmit on all the time slots of the beacon carrier (BCCH carrier on which the BCCH is transmitted). The reason for this lack of capability of transmitting on all time slots of the beacon carrier is an economical one, since it is much cheaper to produce a BTS with reduced transmitting capacity. Stated in other words, if the capacity requirements are so low that only one TCH is needed in the BTS, it is an economically sound idea to produce a low-cost with a hardware limited capability to transmit only on three time slots out of eight.
Those dual-slot BTSs can therefore be derived from a mobile station MS in a cost efficient way, thus reducing the price of the BTS considerably.
However, if only one or two slots within a TDMA-frame having eight time slots are used, there will not be continuous transmission on the beacon carrier (the carrier the BCCH resides).
In general, the mobile station MS has time to monitor in all time slots TS once in the 26-frame multiframe. Stated in other words, once in approximately 120 ms a BTS or BS respectively, that does not send when MS normally listens can be monitored. That is, if the BTS does not transmit in that specific time slot TS the mobile station MS has reserved for monitoring the neighbour cells (i.e. the time slot TS 2 slots after the transmission slot) the mobile station MS has no means to measure the received signal strength from that BTS in a normal way, but has to wait until the 26th frame in the 26-frame multiframe.
This rate, however, is two slow for practical purposes: all the six neighbour base stations BS of the mobile station can only be monitored once in 0.72 s.
It is therefore an object of the present invention to provide a system, in which a mobile station MS can monitor a neighbouring base station BS in each frame, or one in each frame, i.e. once in 4.615 ms, even if the base stations do not transmit continuously on the beacon carrier.
This object is achieved by a system operating according to a method for monitoring at least one base station with discontinuous beacon carrier by at least one mobile station (MS), wherein control information is intermittently transmitted from said at least one base station to said at least one mobile station; traffic information is transmitted between said at least one base station and said at least one mobile station, said at least one base station using n (n being an integer less than 8) time slots of a TDMA frame; and said at least one base station is monitored by said at least one mobile station within a predetermined time slot of the TDMA frame during which said at least one base station transmits information.
More particularly, by synchronization of the transmission of the BS. Which force all mobile stations in the system to monitor at the same time, i.e. at the time of transmission by the BS, the present invention provides a system in which the mobile station can monitor neighbouring base stations in each frame at a rate suitable for practical purposes.
Consequently, a cost effective BTS system of so-called dual slot BTSs can be derived in which the monitoring of neighbouring base stations is satisfactorily achieved at a rate high enough for practical purposes and which can be applied in the field of Home Base Stations.
Additionally, since the BS does not send and/or receive within all time slots, the BS has time to send an additional burst (e.g. dummy burst) at the time the mobile stations MS are monitoring. Therefore, no additional or specially adapted hardware is necessary in the BS to perform the control method according to the invention.